XFA - Add a layer to display XFA forms (#13069)

- add an option to enable XFA rendering if any;
  - for now, let the canvas layer: it could be useful to implement XFAF forms (embedded pdf in xml stream for the background and xfa form for the foreground);
  - ui elements in template DOM are pretty close to their html counterpart so we generate a fake html DOM from template one:
    - it makes easier to translate template properties to html ones;
    - it makes faster the creation of the html element in the main thread.

web/app_options.js

@@ -205,6 +205,11 @@ const defaultOptions = {
     value: "",
     kind: OptionKind.API,
   },
+  enableXfa: {
+    /** @type {boolean} */
+    value: false,
+    kind: OptionKind.API,
+  },
   fontExtraProperties: {
     /** @type {boolean} */
     value: false,